Public attention has been drawn to a solar cell as clean energy source, and its technology has been remarkably developed. In particular, a thin-film solar cell using a photoelectric conversion layer made principally of amorphous silicon is expected to meet the necessary requirements, since the photoelectric conversion layer can be easily formed over a large area at a relatively low cost. A glass substrate used in a conventional thin-film solar cell is of undesirable thickness, heavy and prone to break, and there are increasing requirements for reduced thickness and reduced weight of the substrate, so as to achieve an improved work efficiency, for example, when the solar cell is installed on an outdoor roof. To meet these requirements, a flexible thin-film solar cell using a flexible plastic film or a metallic thin film as a substrate is being put to practical use.
The thin-film solar cell includes electrode layers formed on both surfaces of the photoelectric conversion layer formed on the substrate. One of the electrode layers, located on the side of light incidence, is a transparent electrode layer made of a transparent conductive material, such as ITO or ZnO. Since the transparent electrode layer has a large sheet resistance, flow of electric current through the transparent electrode layer results in increased power loss. In a conventional method, therefore, the thin-film solar cell was divided into a plurality of unit cells having a small width, and each unit cell was electrically connected to its adjacent unit cell to provide a direct series connection structure. As an improvement over this structure, the present inventor proposed a thin-film solar cell as disclosed in U.S. Pat. No. 5,421,908, in which an insulating substrate has holes formed therethrough, which are used to connect the transparent electrode layer disposed on the side of the photoelectric conversion layer remote from the substrate, with a connecting electrode layer on the rear surface of the substrate. This makes it possible to reduce the length of the path of electric current flowing through the transparent electrode layer having a high sheet resistance. Accordingly, a low-voltage, large-current type solar cell can be constructed without dividing it into a number of unit cells having limited dimensions, assuring reduced joule loss, and an increased effective power generating area due no reduction of dead space in the cell.
FIGS. 2a and 2b show a thin-film solar cell corresponding to that shown in FIG. 24 of U.S. Pat. No. 5,421,908 identified above, wherein FIG. 2a is a plane view as seen from the light incidence side of the cell, and FIG. 2b is a plane view showing the rear surface of the substrate. In the interest of clarity, a transparent electrode layer 5, a photoelectric conversion layer 4, and a connecting electrode layer 2 are shown in differently hatched or dotted areas in these figures. Two kinds of connecting holes, that is, first holes 11 and second holes 12 are formed through a flexible substrate 1 and other layers. The first holes 11 are formed through the substrate 1 prior to formation of the other layers, and serve to connect a rear electrode layer disposed between the photoelectric conversion layer 4 and the substrate 1, with the connecting electrode layer 2, by means of parts of the rear and connecting electrode layers deposited on inner walls of the holes 11. The second holes 12 are formed after formation of the rear electrode layer, and serve to connect the transparent electrode layer 5 with the connecting electrode layer 2, by means of parts of the transparent and connecting electrode layers deposited on inner walls of the holes 12. The layers on the substrate 1 are divided into four unit cells by parallel patterning lines 7, and the connecting electrode layer 2 on the rear surface of the substrate 1 is divided by patterning lines 8 which are not aligned with the patterning lines 7 on the front side of the substrate 1. In this arrangement, the unit cells are connected in direct series with each other through the first and second holes 11, 12 and the connecting electrode layer 2. In this solar cell, the photoelectric conversion layer 4 is exposed in its areas around the first holes 11, where a transparent electrode layer is not formed by use of a mask, so that the transparent electrode layer 5 is not connected with the connecting electrode layer 2 through the first holes 11.
FIG. 3 is a cross sectional view showing a thin-film solar cell as shown in FIG. 27 of U.S. Pat. No. 5,421,908. In this solar cell, a metallic electrode layer 3 is connected with a connecting electrode layer 22 on the rear surface of the substrate 1, through a printed electrode 21 embedded in each of the first holes 11. Since the metallic electrode layer 3, photoelectric conversion layer 4 and transparent electrode layer 5 are formed after the printed electrodes 21 are embedded in the first holes 11 formed through the substrate 1, the transparent electrode layer 5 is substantially insulated from the rear electrode layer 3 and printed electrode 21, due to presence of a high resistance layer of an amorphous semiconductor in the photoelectric conversion layer 4. Further, the transparent electrode layer 5 can be formed over the entire area of the major surface of the substrate 1, since the first holes 11 are closed by the printed electrodes 21. The connecting electrode layer 22 is also formed by printing, and is connected with the transparent electrode layer 5, through the thus printed electrodes 22 embedded in the second holes 12. The insulation between the printed electrodes 22 and the rear electrode layer 3 is established by the photoelectric conversion layer 4 deposited in the inner walls of the second holes 12.
In the conventional thin-film solar cell as shown in FIGS. 2a and 2b, the transparent electrode layer 5 has a width "b" of 110 mm, with respect to the width "a" of 130 mm of the substrate 1 as measured in the transverse direction in FIG. 2. The width "c" of the unit cell as measured in the vertical direction in FIG. 2 is 12 mm, and the diameter "d" of the holes 12 is 1 mm, while the width "e" of the patterning lines 7 is 200 .mu.m. Accordingly, the effective photoelectric conversion area of the solar cell is reduced by 15.3%, due to the absence of the transparent electrode layer 5 in the vicinity of the first holes 11, and further reduced by 1.1% and 1.4% due to the second holes 12 and the patterning lines 7, respectively. Consequently, the solar cell only provides a small effective area of photoelectric conversion, which is 82% of the total area of the substrate 1, since the transparent electrode layer 5 is not formed in the vicinity of the first holes 11. Thus, the number of the holes 11 cannot be increased so as to reduce resistance loss in these holes 11.
In the structure of FIG. 3, the transparent electrode layer 5 is formed over the entire area of the substrate, causing no loss in the effective photoelectric conversion area. It is, however, difficult to fill the holes 11, 12 with the printed electrodes, and the size of these holes 11, 12 must be considerably increased so as to facilitate filling of the holes with the electrodes, which results in an increased loss of the effective photoelectric conversion area. Although it may be also considered to form the transparent electrode layer over the entire area of the substrate, and remove only parts of the electrode layer around the first holes, this method is also technically difficult to carry out.